1. Field of the Invention
The present invention relates to a color conversion device for converting a set of input color data into a set of print control data for controlling a printer.
2. Description of the Related Art
Conventionally, when reproducing a color image on print sheet using a printer, a color image of a previously printed sample is first picked up by a scanner to obtain set of color values (R, G, B) where R, G and B represent red, green and blue color components of the sample color image picked up by the scanner. Then, the set of color values G, B) is converted into a set of print control data (C, M, Y, K) where C, M, Y and K represents cyan, magenta, yellow and black components. The printer is controlled by the print control data (C, M, Y, K) to print out a color image corresponding to the color image.
There is the case, however, where the coloring materials (for example, colored toners) used in the printer to reproduce colors are different from the coloring materials used to reproduce the sample color image. In this case, it is impossible to produce a color image that resembles the sample color image.
Recently, networks, incorporating various scanners and printers for processing color images, have become popular in offices and other work areas. However, two different types of devices, for example, two different types of scanners, obtain different values of red (R), green (G), and blue (b) color components, even though picking up the same color image. Similarly, the same colors can not be reproduced by different types of printers even when both are controlled to be driven according to the same print control data (C, M, Y, K). Thus, color images with substantially the same color tones can not be printed out, unless all devices used for color image processes, that is, the scanner by which the color value data is taken and the printer by which the image is printed, are known.
To solve this problem, so-called device independent color systems have been proposed. The device independent color systems attempt to represent colors by color data defined in colorimetric systems determined by the Commission Internationale de L'Eelarage (CIE) and other organizations. In the CIE system, for example, coordinates are assigned to colors according to their appearance under a standard illumination as viewed by a standard observer.
The primaries theory establishes that all colors can be expressed by mixing combinations of red, green, and blue color components. Colors can generally be expressed by a coordinate system made from three axes. Various methods have been provided for expressing colors according to locations of the colors determined by the three axes. Colorimetric systems formed from the three axes are termed color space. Colors can be expressed in the form of coordinate values in the color space. The color space coordinate axes are formed to depend on, for example, the amount of stimulus to the cone cells on the retina of the human eye, and not on the characteristics of a device. In this way, a color index can be produced that does not depend on characteristics of a device. This is the thinking behind the device independent color system.
Although the general colors of a sample can be determined using color spaces, there are practical limits to the amount of information the set of color data can indicate. Because colors of the color image sample can not be indicated with complete accuracy, the outputted colors differ from the sample colors to some degree. Additionally, the distance between two colors within a color space sometimes bears little relationship to the actual difference in the colors as perceived by the human eye. Therefore, a minute difference within the color space can translate into a perception of greatly differing colors. As a result, desired colors may be difficult to reproduce. Many varieties of color space attempt to reduce the difference between a color indicated by the color space and the perception of that color by the human eye. Many color spaces, such as CIE Luv and CIE Lab, have been provided and put into practical application. These types of color spaces have been termed uniform color spaces.
However, even uniform color spaces such as CIE Luv and CIE Lab are unable to completely represent the perception of the human eye. Determining the distortions in MacAdam's color differentiation ellipses (see FIGS. 1 and 3) and the Munsell notation system (see FIGS. 2 and 4) can be used as gauges for judging to what degree a uniform color space represents the perception of the human eye. MacAdam's color differentiation ellipse is appropriate for determining uniformity in a region with an extremely small color difference. On the other hand, the Munsell notation system is appropriate for determining uniformity in a region with an extremely large color difference. MacAdam's color differentiation ellipses indicate uniformity by the degree to which an ellipse is circular. The more circular the ellipse, the greater the uniformity. Uniformity is best in the Munsell notation system the greater the uniform chroma lines, which indicate regions where color chroma is equal, are formed in concentric circles with a uniform interval between the lines, and the straighter the radial equivalent hue lines are and the more evenly they divide 360.degree..
Distortion in a CIE 1976 L*u*v* uniform color space is judged by both the MacAdam's color differentiation ellipses and the Munsell notation system. Results are shown in FIGS. 1 and 2, respectively. Similarly, distortion in a CIE 1976 L*a*b* uniform color space is judged by both the MacAdam's color differentiation ellipses and the Munsell notation system. Results are shown in FIGS. 3 and 4. These results indicate that the CIE 1976 L*a*b* color space is distorted greater than the CIE 1976 L*u*v* color space according to the MacAdam's color differentiation ellipses, and that the CIE 1976 L*u*v* color space is distorted greater than the CIE 1976 L*a*b* color space according to the Munsell notation system. These results therefore show that the CIE 1976 L*u*v* color space is more uniform than the CIE 1976 L*a*b* color space, in their regions with extremely small color differences, and the CIE 1976 L*a*b* color space is more uniform than the CIE 1976 L*u*v* color space in their regions with extremely large color differences. These results have been described by Mitsuo Ikeda in "Fundamentals of Color Engineering", Asakura Publishers, published on Sep. 20, 1980.
Thus, there are minute shifts and distortions in the uniform color spaces in regards to the perception of the human eye. Performing color conversion without considering these distortions translates into reproductions of images that have shifts and distortions in regards to the perception human eye. This produces the problem wherein desired images can not be accurately obtained in regards to the human visual sense.